The present invention relates to a treatment of flue gas containing sulfur oxides, more particularly to a flue gas treatment method and apparatus for removing sulfur oxides by injecting ammonia into flue gas containing sulfur oxides.
As economy develops, more and more energy is demanded. In such circumstances, an energy source is still dependent on fossil fuels such as coal and petroleum. However, harmful products or pollutants generated by burning of fossil fuels are responsible for environmental pollution. To prevent the diffusion of pollutants such as sulfur oxides and nitrogen oxides into the atmosphere and the progress of environmental pollution, development work of a fuel gas treatment system is being carried out for a fuel combustion plant such as a power plant. However, in the conventional flue gas treatment system, there are still many areas of improvement to meet problems such as the need for the equipment requiring complicated control and the indispensability for large-scale waste water treatment systems.
In order to solve these problems, a flue gas desulfurizing process in which flue gas discharged from the combustion facility such as a boiler is treated by injecting ammonia into the flue gas has been developed.
In the flue gas desulfurizing process in which ammonia (NH3) is injected into flue gas containing sulfur oxides (SOX) such as combustion gas discharged from a boiler (hereinafter referred to as an ammonia injection process), NH3 reacts with SOX to produce a powder of ammonium compounds containing ammonium sulfate. Among reactions in the ammonia injection process, a chemical reaction in which sulfur dioxides (SO2) as a main component of SOX reacts with NH3, oxygen (O2) and water (H2O) contained in flue gas to produce ammonium sulfate [(NH4)2SO4] as by-product is expressed in the following formula (1).
SO2+2NH3+H2O+1/2O2xe2x86x92(NH4)2SO4+550.1KJ/molxe2x80x83xe2x80x83(1)
As expressed in the formula (1) representatively, because the reaction in which SOX reacts with NH3 to produce ammonium compounds is an exothermic reaction, the lower the temperature of flue gas, the more accelerated the reaction is. Therefore, in the ammonia injection process, flue gas is cooled before injection of NH3, or water is sprayed into flue gas before, or simultaneously with, or after injection of NH3; or after mixing with NH3. In this case, the injected water is consumed by the desulfurizing reaction representatively expressed in the formula (1), and evaporated by the heat of reaction, and the sensible heat possessed by flue gas before injection of ammonia. Thus, if the amount of sprayed water is adjusted appropriately, the recovery of the produced ammonium compounds in the form of powder is not hindered. The recovery of the powder is normally carried out in an electrostatic precipitator, and the recovered powder comprises ammonium compounds such as ammonium sulfate and can be utilized as a fertilizer.
However, in the ammonia injection process, generally, efficiency of removing SOX (hereinafter referred to as desulfurization rate), especially efficiency of removing SO2 is not high. Residual NH3 which has injected and has not reacted with SOX is released together with the treated flue gas to the atmosphere. In order to lower leakage of NH3, it is necessary to reduce the amount of injected ammonia. However, reduction of the amount of injected ammonia lowers further a desulfurization rate, particularly the efficiency of removing SO2. As a result, unreacted NH3 is additionally released by the amount corresponding to the lowered desulfurization rate. As a result, the problem of leakage of NH3 which is not lowered by the amount corresponding to the reduced amount of injected ammonia arises.
On the other hand, the desulfurizing reaction may be accelerated by increasing the amount of water sprayed together with NH3 to lower the temperature of flue gas. In this case, the temperature of the flue gas reaches around a water saturation temperature even in the vicinity of an outlet of a process vessel, and hence it becomes difficult to recover the produced powder in a dry state.
Thus, in order to achieve a high desulfurization rate, normally, NH3 is sprayed and injected, and irradiation of electron beam of several kGy to dozen kGy is carried out (the flue gas desulfurizing method in which injection of NH3 and irradiation of electron beam are performed is hereinafter referred to as electron beam process). The purpose of this method is that residual SO2 which has not been removed in the above formula (1) is oxidized to sulfur trioxide (SO3) or sulfuric acid (H2SO4) by radicals such as O, OH or HO2 produced from gas molecules such as oxygen and water vapor in flue gas by irradiation of electron beam, and then the produced SO3 or H2SO4 reacts with water (water vapor originally contained in the flue gas, and water sprayed and injected together with NH3) and NH3 by the following formulas (2) and (3) to recover ammonium sulfate.
SO3+2NH3+H2Oxe2x86x92(NH4)2SO4xe2x80x83xe2x80x83(2)
H2SO4+2NH3xe2x86x92(NH4)2SO4xe2x80x83xe2x80x83(3)
In order to irradiate flue gas of weight flow Q (kg/s) with electron beam having absorbed dose of D (kGy), electric power P (kW) calculated by the following formula (4) is consumed.
P(kW)=Q(kg/s)xc3x97D(kGy)/(xcex7(%)/100)xe2x80x83xe2x80x83(4)
where xcex7 is the ratio of energy of electrons absorbed by the flue gas to the supplied electric power, and this xcex7 is normally in the range of 50 to 80%.
However, generally, in the electron beam irradiating method, when a high desulfurization rate is required on the condition that leakage of NH3 is controlled to a lower level, the required absorbed dose becomes large, and hence electric power consumption becomes large as expressed by the formula (4).
Therefore, the inventors of the present application have proposed a flue gas desulfurizing method and apparatus in which flue gas is cooled in the range of a water saturation temperature to 80xc2x0 C., aqueous ammonia is sprayed and injected into the cooled flue gas, and the aqueous ammonia is pulverized into droplets having a Sauter mean diameter of 0.5 xcexcm to 30 xcexcm and sprayed, whereby a high desulfurization rate is achieved while keeping low leakage of NH3 without irradiation of electron beam or with a relatively small absorbed dose.
However, even in such method, in order to obtain 90% or more of the desulfurization rate while leakage of NH3 is suppressed to about 10 ppm or less, a large amount of absorbed dose of not less than about 5 kGy is required, or aqueous ammonia is required to be pulverized into droplets having a Sauter mean diameter of not more than about 5 xcexcm. In the latter as well as the former, a large amount of energy such as energy for generating a large amount of compressed air for pulverization is necessary.
In view of the above, it is therefore an object of the present invention to provide a flue gas desulfurizing method and apparatus which can achieve a high desulfurization rate and can lower leakage of NH3 while reducing the cost of energy.
In order to achieve the above object, according to the present invention, there is provided a flue gas treatment method for removing sulfur oxides in flue gas using ammonia, characterized in that: ammonia is injected into the flue gas containing sulfur oxides to react sulfur oxides with ammonia to produce ammonium compounds containing ammonium sulfate, and after recovering the produced ammonium compounds from the flue gas, the flue gas is brought into contact with an absorption liquid to remove residual sulfur oxides and/or ammonia contained in the flue gas.
In the absorption liquid, sulfate ions (SO42xe2x88x92) and/or sulfite ions (SO32xe2x88x92) and/or ammonium ions (NH4+) are dissolved.
In the flue gas treatment method of the present invention, the residual SOX, contained in the flue gas, which has not reacted with the injected NH3 and has not converted into a powder of ammonium compounds, and/or the residual NH3 which has injected into the flue gas, and has not reacted with SOX and has not converted into a powder of ammonium compounds are brought into contact with an absorption liquid in which the above SO42xe2x88x92 and/or SO32xe2x88x92 and/or NH4+ is dissolved, and are absorbed in the absorption liquid and removed.
In the flue gas treatment method of the present invention, in the case where the flue gas treatment capability (desulfurization rate and concentration of leak ammonia) at the section of an ammonia injection process at an upstream side except a gas absorption apparatus is the same as that of the ammonia injection process only, the flue gas treatment capability of the entire flue gas treatment system including the gas absorption apparatus can be improved further than that of the ammonia injection process only. That is, the desulfurization rate of the entire flue gas treatment apparatus can be improved by absorbing and removing SOX in the gas absorption apparatus. Further, the concentration of leak ammonia in the entire flue gas treatment apparatus can be lowered by absorbing and removing NH3 in the gas absorption apparatus.
When the flue gas treatment capability of the entire flue gas treatment apparatus is the same as that of the section of the ammonia injection process only, the running cost and the equipment in the section of the ammonia injection process can be significantly reduced. Specifically, the desulfurization rate which should be achieved at the section of the ammonia injection process can be lowered by absorbing and removing SOX in the gas absorption apparatus. Further, the requirement for concentration of leak ammonia at the section of the ammonia injection process can be alleviated by absorbing and removing NH3 in the gas absorption apparatus. As a result, the flue gas treatment capability which has been achieved by irradiation of a relatively large amount of electron beam in the conventional technology can be achieved by irradiation of a small dose of electron beam or no irradiation of electron beam. Furthermore, the flue gas treatment capability which has been achieved by pulverizing aqueous ammonia into droplets having an extremely small Sauter mean diameter and spraying such droplets in the conventional technology can be achieved even in spraying aqueous ammonia having a relatively large diameter. In this manner, the running cost and the cost of equipment can be extremely reduced in the section of the ammonia injection process.
According to one aspect of the present invention, the absorption liquid is circulated and used while oxidizing sulfite ions dissolved in the absorption liquid. Specifically, NH3 is injected into flue gas to react with SOX , and the flue gas from which reaction product has been recovered is brought into contact with the absorption liquid which is circulated and used while oxidizing dissolved SO32xe2x88x92 to SO42xe2x88x92, thus removing NH3 and SOX simultaneously.
If aqueous solution in which NH4+ and SO32xe2x88x92 are dissolved is compared with aqueous solution in which NH4+ and SO42xe2x88x92 are dissolved, when the molar ratio of NH4+ to SO32xe2x88x92 is equal to the molar ratio of NH4+ to SO42xe2x88x92, a pH of the aqueous solution in which NH4+ and SO42xe2x88x92 are dissolved is lower than that of the aqueous solution in which NH4+ and SO32xe2x88x92 are dissolved. For instance, in the case of NH4+:SO32xe2x88x92=NH4+:SO42xe2x88x92=2:1, the respective concentrations of the aqueous solution are shown in the following Table 1 according to the sum of the concentrations of the salts.
If the NH4+ concentration in the absorption liquid is the same, as a pH of the absorption liquid is lower, NH3 in the flue gas is liable to be dissolved in the absorption liquid. Thus, the residual NH3 in the flue gas from which the reaction product has been recovered can be effectively removed by oxidizing SO32xe2x88x92 in the absorption liquid to SO42xe2x88x92.
While one of main components of SOX which remains in the flue gas after recovering the reaction products is SO2, the SO32xe2x88x92 concentration in the absorption liquid has a great influence on absorption of SO2 rather than a pH of the absorption liquid as far as the pH of the absorption liquid is not lowered remarkably. Specifically, if the pH of the absorption liquid is about 2 or more, then the removal efficiency of SO2 by the absorption liquid is not much influenced by the pH of the absorption liquid. However, the removal efficiency of SO2 is lowered, when the SO32xe2x88x92 concentration is high. Thus, SO32xe2x88x92 in the absorption liquid is oxidized to SO42xe2x88x92 to remove SO2 effectively which remains in the flue gas after recovering the reaction product. The oxidization of SO32xe2x88x92 in the absorption liquid to SO42xe2x88x92 is performed by, for example, diffusion of air into the absorption liquid (aeration).
According to one aspect of the present invention, the amount of the ammonia injected into the flue gas is adjusted so that a pH of the absorption liquid becomes 8 or less without supplying acid substance.
In the case where NH4+ is dissolved together with SO32xe2x88x92 and/or SO42xe2x88x92 in an aqueous solution, if the molar ratio of NH4+ to SO32xe2x88x92 and/or SO42xe2x88x92 is equal to or smaller than 2:1, then NH4+ can be in existence relatively stably as ions in the aqueous solution. Particularly, when SO32xe2x88x92 is coexistent with NH4+, if the molar ratio of NH4+ to SO32 becomes larger than 2:1, then NH4+ in the aqueous solution can be easily liberated as NH3 gas.
Therefore, if the molar ratio of NH4+ to SO32xe2x88x92 in the absorption liquid becomes larger than 2:1, efficiency for dissolving and removing NH3 in the flue gas into the absorption liquid is lowered. According to Table 1, because the pH of the absorption liquid is about 8 in the case where the molar ratio of NH4+ to SO32xe2x88x92 is 2:1, it is preferable to adjust the pH of the absorption liquid to 8 or less in order to effectively remove NH3 remaining in the flue gas from which the reaction product has been recovered.
It is possible to adjust the pH of the absorption liquid to 8 or less by supplying acid substance such as sulfuric acid into the absorption liquid. In this case, additional chemicals are necessary to remove SOX in the flue gas besides NH3, and hence the running cost is increased and additional facilities such as chemical storage equipment are necessary.
It is also possible to adjust the pH of the absorption liquid to 8 or less by increasing the amount of the absorption liquid withdrawn from a gas absorption apparatus and increasing the amount of industrial water supplied to the absorption liquid in accordance with the amount of the withdrawn liquid and the amount of evaporated water in the gas absorption apparatus to lower the concentration of the salt in the absorption liquid. However, as shown in Table 1, because the suppression effect of pH obtained by lowering the concentration of the salt is extremely limited, if the pH is lowered by increasing the amount of withdrawn absorption liquid, then a large amount of the absorption liquid is required to be withdrawn, and hence the great equipment cost and running cost are required to treat the withdrawn water.
On the other hand, when flue gas which is brought into contact with the absorption liquid in the gas absorption apparatus contains not only unreacted NH3 but also SOX, the absorption liquid absorbs both of NH3 and SOX simultaneously to increase a concentration of NH4 and a concentration of SO32xe2x88x92 and/ or SO42xe2x88x92. Thus, the pH of the absorption liquid can be adjusted by adjusting a ratio of a concentration of SOX to a concentration of NH3 adequately in the flue gas introduced into the gas absorption apparatus.
The concentrations of SOX and NH3 remaining in the flue gas introduced into the gas absorption apparatus are determined by a flue gas treatment capability (desulfurization rate and concentration of leak ammonia) which is achieved at the section of the ammonia injection process at the upstream side except the gas absorption apparatus. The flue gas treatment capability achieved at the section of the ammonia injection process is determined by process variables such as the amount of ammonia to be injected, a flue gas temperature at an outlet of a process vessel, and the dose of electron beam to be irradiated, if electron beam is irradiated along with NH3 injection. The most suitable process variable among the above variables for adjusting a ratio of a concentration of SOX to a concentration of NH3 remaining in the flue gas is the amount of NH3 to be injected. Therefore, the pH of the absorption liquid can be adjusted by the amount of NH3 to be injected.
Specifically, when the amount of injected NH3 is large, a rate for converting SOX into ammonium compounds containing ammonium sulfate is increased by a reaction of SOX with the injected NH3 to thus decrease the concentration of the remaining SOX while the concentration of unreated NH3 is increased. As a result, the amount of SOX dissolved in the absorption liquid is decreased and the amount of NH3 dissolved in the absorption liquid is increased by bringing the flue gas into contact with the absorption liquid. Consequently, the pH of the absorption liquid is increased. Conversely, when the amount of injected NH3 becomes small, the concentration of SOX remaining in the flue gas which is brought into contact with the absorption liquid is lowered and the concentration of unreacted NH3 is increased, thus lowering the pH of the absorption liquid. In this manner, it is possible to adjust the pH of the absorption liquid by adjusting the amount of NH3 to be injected, and if the amount of NH3 to be injected into the flue gas is adjusted so that the pH of the absorption liquid is adjusted to 8 or less, then the pH of the absorption liquid can be adjusted to 8 or less without additional chemicals of acid substance and without withdrawing a large amount of the absorption liquid.
As described later, alkaline substance such as NH3 may be added to the absorption liquid. In such case also, it is preferable to adjust the amount of NH3 to be injected into the flue gas at the section of the ammonia injection process so that the pH of the absorption liquid is adjusted to 8 or less without adding acid substance and adjust the amount of alkaline substance to be added that the pH of the absorption liquid becomes 8 or less. At this time, if the variable range of pH caused by injection of alkaline substance is considered, it is particularly preferable that the amount of NH3 injected into the flue gas is adjusted so that the pH of the absorption liquid is adjusted to 7 or less without adding alkaline substance and the amount of supplied alkaline substance composed of NH3 is adjusted so that the pH of the absorption liquid is adjusted to 8 or less.
According to one aspect of the present invention, the pH of the absorption liquid is adjusted by supplying ammonia into the absorption liquid. If the pH of the absorption liquid is less than 2, even though SO32xe2x88x92 in the absorption liquid is oxidized into SO42xe2x88x92, absorption efficiency of SO2 in SOX is lowered. Accordingly, alkaline substance may be supplied into the absorption liquid. In this cage, by utilizing a part of NH3 which is used for removing SOX in the flue gas, it is advantageously unnecessary to provide additional equipment such as a storage apparatus for storing alkaline substance for replenishment.
In the above flue gas treatment method, particularly, if the concentration of SOX is much higher than the concentration of NH3 at the inlet of the gas absorption apparatus, the pH of the absorption liquid is lowered, and hence the desulfurization rate in the absorption apparatus cannot be increased. Thus, the SOX concentration at the inlet of the gas absorption apparatus cannot be increased excessively compared to the SOX concentration achieved in the entire gas treatment apparatus. Therefore, the lowering effect of the cost of equipment and the running cost at the section of the ammonia injection process is limited.
However, this problem can be solved by adding NH3 into the absorption liquid which is circulated and used. Specifically, even though the desulfurization rate at the section of the ammonium injection method is extremely lowered and the SOX concentration is much higher than the concentration of NH3 at the inlet of the gas absorption apparatus, the pH is adjusted to 2 or more by supplying NH3 into the absorption liquid circulated and used in the gas absorption apparatus to increase a desulfurization rate in the gas absorption apparatus, with the result that a desired desulfurization rate can be achieved as an entire flue gas treatment apparatus.
Furthermore, if material other than NH3 is used as alkaline substance, cation, other than NH4+ which is inevitably dissolved in the absorption liquid from the flue gas, makes treatment for the withdrawn absorption liquid complex. In contrast thereto, according to the flue gas treatment method of the present invention, after SO32xe2x88x92 is oxidized to SO42xe2x88x92 in the absorption liquid, water content in the withdrawn liquid is evaporated, whereby ammonium compounds containing ammonium sulfate can be obtained and utilized as a fertilizer together with a powder obtained in the section of the ammonia injecting method.
According to one aspect of the present invention, supply of ammonia into the absorption liquid is performed by diffusion of ammonia gas into the absorption liquid.
While it is possible to supply NH3 into the absorption liquid in the form of aqueous ammonia, NH3 is normally supplied in the form of ammonia gas into an ammonia injection apparatus in the ammonia injection process. Therefore, if NH3 is supplied into the absorption liquid by diffusion of ammonia gas into the absorption liquid, an ammonia gas supply apparatus can be advantageously shared. The diffusion of ammonia gas into the absorption liquid may be performed by using a diffusion pipe composed of porous material and having many fine pores to dissolve ammonia gas efficiently into an object liquid for adjusting a pH. This porous pipe may be made of ceramic materials, specifically alumina porcelain. The diameters of the pores are preferably in the range of 10 to 50 xcexcm.
According to one aspect of the present invention, the flue gas is brought into contact with cooling water before injecting ammonia into the flue gas, and a part of the cooling water which has contacted the flue gas is withdrawn, and then the withdrawn water is supplied as it is or after the dissolved sulfite ions in the withdrawn water are oxidized, as make-up water of the absorption liquid. Specifically, before injecting NH3 into the flue gas containing SOX, the flue gas is brought into contact with the cooling water while ammonium compounds produced by a reaction of SOX with NH3 are recovered, and then the flue gas is brought into contact with the absorption liquid whose make-up water is a part of the above cooling water to remove NH3 contained in the flue gas.
By bringing the flue gas containing SOX into contact with the cooling water, a part of SOX in the flue gas is dissolved as SO32xe2x88x92 or SO42xe2x88x92 in the cooling water, and hence the pH of the cooling water is lowered to less than 7. On the other hand, when the flue gas which has been discharged from the section of the ammonia injection process is brought into contact with the absorption liquid to absorb unreacted NH3, the NH4+ concentration in the absorption liquid is gradually increased, and thus the pH of the absorption liquid is increased. If the cooling water which has contacted the flue gas is used as the make-up water whose amount corresponds to the amount of the withdrawn water plus the amount of the evaporated water being supplied to the absorption liquid, an increase of the pH of the absorption liquid can be suppressed. As described above, when the pH of absorption liquid is increased, absorption efficiency of NH3 by the absorption liquid is lowered. Thus, when the cooling water which has contacted the flue gas is used as make-up water, a lowering of the absorption efficiency of NH3 in the flue gas by the absorption liquid can be suppressed.
According to one aspect of the present invention, before injecting the ammonia into the flue gas, the flue gas is brought into contact with a heat exchange surface which is cooled to a temperature of not more than water saturation temperature in the flue gas, and then the condensed water generated on the heat exchange surface is withdrawn partially or entirely, and the withdrawn water is supplied as make-up water of the absorption liquid as it is or after the dissolved sulfite ions in the withdrawn water are oxidized. Specifically, before injecting NH3 containing SOX into the flue gas, the flue gas is brought into contact with the heat exchange surface which is cooled to a temperature of not more than water saturation temperature in the flue gas, while ammonium compounds produced by a reaction of SOX with NH3 are recovered, and then the flue gas is brought into contact with the absorption liquid whose make-up water is a part of or whole of the condensed water to remove NH3 contained in the flue gas.
When the flue gas containing SOX is brought into contact with the heat exchange surface which is cooled to a temperature of not more than water saturation temperature in the flue gas, a part of water content contained in the flue gas is condensed and a part of SOX in the flue gas is dissolved as SO32xe2x88x92 or SO42xe2x88x92 in the condensed water, and hence the pH of the condensed water is lowered to less than 7. Therefore, if the condensed water is used as make-up water of the absorption liquid, an increase of the pH of the absorption liquid is suppressed, and a lowering of the absorption efficiency of NH3 in the flue gas by the absorption liquid can be prevented.
As described above, the effect of lowering the pH in the case where SOX is dissolved in the absorption liquid in the form of SO42xe2x88x92 is greater than that in the case where SOX is dissolved in the form of SO32xe2x88x92, and hence the effect of suppression of lowering the absorption efficiency of NH3 when SO42xe2x88x92 is dissolved is also large. Therefore, SO32xe2x88x92 in the cooling water or the condensed water is oxidized to SO42xe2x88x92 by a means such as aeration, and this cooling water is used for make-up water to thus increase the effect of suppression of lowering the absorption efficiency of NH3 remarkably.
In the meantime, when flue gas is cooled by bringing the flue gas into contact with cooling water circulated and used before injecting NH3 into the flue gas at the section of the ammonia injection process, a pH of the recirculating cooling water is extremely lowered or SS concentration of the recirculating cooling water is extremely increased to cause a gas cooling apparatus not to be operated stably. In order to prevent this problem, a part of the recirculating cooling water is required to be withdrawn, and if the withdrawn recirculating cooling water is not supplied to a gas absorption tower as make-up water, such water is necessary to be discharged after separate treatment. In the case where flue gas is brought into contact with the heat exchange surface which is cooled to a temperature of not more than water saturation temperature in the flue gas before injecting NH3 into the flue gas, as described above, the condensed water generated on the heat exchange surface is required to be discharged after separate treatment, if the condensed water is not supplied to the gas absorption tower as make-up water. In this case, because a pH of the withdrawn water and the condensed water is normally very low, adjustment of the pH using alkaline substance is an indispensable process.
According to the present invention, the withdrawn water or the condensed water is supplied to a gas absorption apparatus as make-up water and the pH is adjusted by NH3 absorbed in the flue gas in the gas absorption apparatus and injecting alkaline substance such as NH3, and thus it is not necessary to provide an additional pH adjusting apparatus as mentioned above. Therefore, this is effective in reducing the cost of equipment. Especially, if alkaline substance other than NH3 is not supplied to the absorption liquid, and ammonium compounds containing ammonium sulfate are obtained from the withdrawn water of the absorption liquid as mention above, SO32xe2x88x92 and/or SO42xe2x88x92 contained in the withdrawn water or the condensed water is also contained in a part of the ammonium compounds to thus be utilized as a part of a fertilizer. This is preferable from a viewpoint of effective utilization of resources.
According to one aspect of the present invention, a part of the absorption liquid is withdrawn, and the withdrawn water is sprayed into the flue gas before, or simultaneously with, or after injection of ammonia; or after mixing the withdrawn water with ammonia. In this method, the whole of or a part of water content in the sprayed withdrawn liquid is consumed by a desulfurizing reaction and is evaporated by the heat of reaction and the sensible heat possessed originally by the flue gas. As a result, the whole of or a part of dissolved components in the withdrawn liquid is evaporated, dried and solidified, and recovered together with a powder produced by a reaction of SOX in the flue gas with the injected NH3 by a product recovering apparatus such as an electrostatic precipitator. If the temperature of the flue gas before injection of ammonium, the amount of the withdrawn absorption liquid, and the amount of the sprayed absorption liquid are properly controlled, then the whole of the withdrawn liquid can be evaporated, dried and solidified. When a part of the withdrawn liquid is sprayed or a part of the sprayed withdrawn liquid is not evaporated and becomes waste water, a heat source such as steam is supplied to the remaining withdrawn liquid from the outside to evaporate water content, thereby recovering a powder or solid of ammonium compounds containing ammonium sulfate. In this case, the cost of the equipment and the running cost can be lower compared to treatment of the whole amount of the withdrawn water.
As a method for spraying the withdrawn water, a withdrawn water spray apparatus may be provided besides an ammonia injection apparatus for injecting NH3 into the flue gas, and the withdrawn water spray apparatus may be arranged upstream of, or at the same position as, or downstream of the ammonia injection apparatus. In order to improve the desulfurization rate at the section of the ammonia injection process at the upstream side, it is preferable to mix NH3 with the withdrawn liquid, and spray and inject an ammonia dissolved liquid in the form of drop lets. According to this method, a desulfurization reaction is extremely accelerated in the gas-liquid interfaces between droplets of the ammonia dissolved liquid and gas.
According to one aspect of the present invention, the flue gas is irradiated with electron beam after the ammonia is injected and before the ammonium compounds are recovered.
In the flue gas treatment method of the present invention, in the case where the flue gas treatment capability (desulfurization rate and concentration of leak ammonia) at the section of the electron beam process at an upstream side except a gas absorption apparatus is the same as that of the electron beam process only, the flue gas treatment capability of the entire flue gas system including the gas absorption apparatus can be improved further than that of the electron beam process only. When the flue gas treatment capability of the entire flue gas treatment apparatus is the same as that of the electron beam process only, the dose of electron beam required for achieving a desired desulfurization rate can be suppressed remarkably. Thus, the running cost and the cost of equipment in the section of the electron beam process can be significantly reduced.
According to one aspect of the present invention, the electron beam is applied to the flue gas through a metal foil, and the metal foil is cooled with air, and then the air after cooling is injected into the flue gas before the flue gas contacts the absorption liquid.
An electron beam generating apparatus comprises a direct current high voltage power supply for generating direct current high voltage ranging from hundreds kV to several MV, a supply path such as a high-voltage cable for supplying such direct current, and an electron accelerator for accelerating and emitting electrons by the direct current high voltage. The interior of the electron accelerator is kept under vacuum, and a vacuum space is partitioned from the atmosphere by a thin metal foil (hereinafter referred to as window foil). The accelerated electrons pass through the window foil of the electron accelerator side, or in some cases, through this window foil and another window foil for isolating the flue gas from the atmosphere (hereinafter referred to as window foil of the process vessel side) into the flue gas. In this case, because a part of energy of accelerated electrons is lost by the thin metal foil to convert such energy to heat, cooling air is blown on the metal window foil to prevent the temperature of the metal window foil from being raised to a temperature exceeding an allowable temperature (350xc2x0 C. or less). At this time, since air is also irradiated with electron beam, ozone is generated in the cooling air too. In this manner, ozone-containing air thus generated is injected between a product recovering apparatus and a gas absorption apparatus to convert remaining NO to NO2 or N2O5. As a result, as described later, the converted NO2 or N2O5 is absorbed in the absorption liquid according to a pH of the absorption liquid in the gas absorption apparatus.
According to one aspect of the present invention, ozone-containing gas is injected into the flue gas before the flue gas contacts the absorption liquid.
In the case where flue gas such as boiler combustion flue gas contains not only SOX but NOX (mostly NO), and is irradiated with electron beam at the upstream side of the gas absorption apparatus in addition to injection of NH3, besides SOX, NOX is converted to a powder whose main component is ammonium nitrate, and recovered and removed in the product recovering apparatus. However, when the dose of electron beam is small, the rate of removing NOX becomes small, and NO of a part of NOX, and NO2 or N2O5 of the rest of NOX enter the gas absorption apparatus. When irradiation of electron beam is not performed at the upstream side of the gas absorption apparatus, a part of NO2 and N2O5 contained originally in the boiler flue gas is converted to a powder whose main component is ammonium nitrate, and the rest of N2 and N2O5, and most of NO contained in the flue gas enter the gas absorption apparatus without causing reaction. In the gas absorption apparatus, NO2 or N2O5 is absorbed in the absorption liquid according to the pH of the absorption liquid and is converted to nitrite ions (NO2xe2x88x92) or nitrate ions (NO3xe2x88x92), but NO is not mostly absorbed.
Thus, in the case where the dose of electron beam is reduced, or irradiation of electron beam is not performed at all, in order to achieve a high NOX removal rate (hereinafter referred to as denitration rate) in the entire flue gas treatment apparatus, NO is needed to be converted to NO2 or N2O5 at the inlet of the gas absorption apparatus as much as possible. As a method for converting NO to NO2 or N2O5 besides the electron beam irradiation method, there is a corona discharge method. Therefore, it is desirable to employ an electrostatic precipitator utilizing corona discharge as a product collecting apparatus in the section of the electron beam process at the upstream side.
Further, NO may be converted to NO2 or N2O5 by injecting ozone into the flue gas. While ozone may be injected anywhere as far as such injection is performed at the upstream side of the gas absorption apparatus, ozone is preferably injected at the place where NH3 concentration is the smallest in the section of the ammonia injection process at the upstream side, i.e. the place between the product recovering apparatus and the gas absorption apparatus, because ozone reacts with NH3 to produce NOX and to allow itself decomposed. The thus produced NO2 or N2O5, if irradiation of electron beam is performed at the upstream side of the gas absorption apparatus, is absorbed together with NO2 or N2O5 produced by irradiation of electron beam in the absorption liquid, thus becoming nitrite ions (NO2xe2x88x92) and/or nitrate ions (NO3xe2x88x92). In this case, if the gas absorption apparatus has a mechanism for oxidizing SO32xe2x88x92 to SO42xe2x88x92, this NO2xe2x88x92 is also oxidized to NO3xe2x88x92.
According to the present invention, there is provided a flue gas treatment apparatus for removing sulfur oxides in flue gas using ammonia, comprising: an ammonia injection apparatus for injecting ammonia into flue gas containing sulfur oxides, a process vessel for allowing injected ammonia to react with sulfur oxides; a recovering apparatus for recovering produced ammonium compounds containing ammonium sulfate; and a gas absorption apparatus for bringing the flue gas after recovery into contact with an absorption liquid.
According to one aspect of the present invention, the gas absorption apparatus comprises a mechanism for recirculating the absorption liquid, and a mechanism for oxidizing sulfite ions in the absorption liquid.
According to one aspect of the present invention, the gas absorption apparatus comprises a means for adjusting a pH of the absorption liquid by injecting ammonia into the absorption liquid.
According to one aspect of the present invention, the pH adjusting means comprises a supply port for supplying aqueous solution before adjusting a pH of the aqueous solution; a discharge port for discharging the aqueous solution after adjusting a pH of the aqueous solution; a pH adjusting tank having an ammonia diffusing means for diffusing ammonia gas into the aqueous solution held in the tank; a pH measuring device for measuring a pH of the aqueous solution held in the pH adjusting tank; and an ammonia gas supply line having an adjusting valve for supplying ammonia gas to the ammonia diffusing means.
According to one aspect of the present invention, the ammonia injection means comprises a diffusing pipe made of porous material for diffusing the ammonia gas into the absorption liquid.
According to one aspect of the present invention, the flue gas treatment apparatus comprises a gas cooling apparatus for bringing the flue gas containing sulfur oxides into contact with cooling water before injecting ammonia; a means provided in the gas cooling apparatus for circulating and using the cooling water; and a line for withdrawing a part of the cooling water; wherein the line for withdrawing a part of the cooling water is connected to a line for introducing make-up water of the gas absorption apparatus.
According to one aspect of the present invention, the flue gas treatment apparatus comprises a heat exchanger having a heat exchange surface cooled to a temperature of not more than water saturation temperature in the flue gas containing sulfur oxides before injecting ammonia; and a line for withdrawing condensed water generated on a gas-contact surface of the heat exchanger, wherein the line for withdrawing the condensed water is connected to a line for introducing make-up water of the gas absorption apparatus.
According to one aspect of the present invention, the gas absorption apparatus has a line for withdrawing the absorption liquid, and a withdrawn liquid spray apparatus for spraying and injecting the withdrawn absorption liquid is provided upstream of, or at the same position as, or downstream of the ammonia injection apparatus.
According to one aspect of the present invention, the process vessel has a window for allowing electron beam to pass therethrough and irradiating the flue gas therein with electron beam.